1. Field of the Invention
This invention relates to an optical recording medium suited for writing a record with a laser, especially with a semiconductor laser, and more particularly to an improved optical recording medium used in the art of optical disk.
2. Description of the Prior Art
The optical disk comprises generally a circular substrate overlaid with a thin recording layer, on which high density information can be stored in the form of spiral or circular track of minute, optically detectable pits (e.g. about 1.mu. size). For writing information on such a disk, a laser-responsive layer, i.e. the thin recording layer, is scanned spirally or circularly with a convergent laser beam to form pits on the track of the scanning laser beam. The laser-responsive layer can form optically detectable pits by absorbing laser energy. In a heat mode recording system, for instance, the laser-responsive layer can absorb thermal energy of a laser beam and form minute pits at irradiated spots through evaporation or melting. In another heat mode recording system, pits having an optically detectable difference of optical density can be formed on spots wherein laser energy is absorbed.
The information recorded on the optical disk can be scanned along the track of pits with a laser to read out optical variations due to the presence and absence of pit. For instance, the energy of the reading laser reflected from the disk is monitored with a photodetector. When falling on the pit-free area, an output of the photodetector is lowered. On the contrary, when irradiating a pit, the laser is adequately reflected from an underlying reflecting surface, giving a high photodetector output.
In the past, various films were proposed as recording media to be used for these optical disks. Such films are chiefly inorganic materials including thin films of metals such as vacuum-deposited aluminum, thin films of bismuth and of tellurium oxide, and amorphous glass films of chalcogenite group. Generally, these films are responsive to rays of about 350-800 nm in wavelength and have high reflectance for laser beams, thus involving drawbacks such as a low efficiency of laser energy utilization.
For this reason and others, extensive studies have been made in recent years on organic thin films capable of altering some optical properties by irradiation with relatively long wavelength light (longer than about 780 nm). Such organic thin films are effective in that pits can be formed therein by means of a semiconductor laser of wavelength, for example, around 830 nm.
However, organic compounds having absorption maxima in a longer wavelength region are generally unstable to heat and additionally have some technical difficulties with respect to their sublimation property. Such being the case, it cannot be said that an organic thin film satisfactory in performance characteristics has been developed.